Field of the Invention
Multi-layered printed circuit boards involve the use of layers of copper and polymeric dielectric material. These multi-layered circuit boards are in great demand for use in complex electronic devices.
For good serviceability, it is necessary that the circutized copper layer(s) adhere to the polymeric dielectric material. This adhesion, however, is often a commonly found point of failure for those products. Various methods have been used to improve adhesion.
Many of the methods used to improve adhesion put an oxide coating on the copper, improving adhesion between the copper and the polymeric dielectric layer. For example, U.S. Pat. No. 2,460,896 describes a composition for forming a copper oxide coating on copper; the composition including sodium or potassium chlorite nd an alkyli metal hydroxide. These two materials are used in relative proportions within the range of about one part chlorite per part of hydroxide to one part chlorite for each 200 parts of the hydroxide.
The adhesion provided by such early oxide coatings, however, was inferior. The first oxides, usually black and velvety in appearance, would provide a surface having marginal adhesive performance. The long dentrites of the coatings were easily crushed during lamination and frequently resulted in lower bond strengths between the copper and dielectric material when compared to multi-layered boards having no oxide coating.
More recently developed oxide coatings and coating materials are described in U.S. Pat. No. 4,409,037 which describes a composition and process for improving the adhesion of copper foil to polymeric printed circuit substrates. The composition comprises an aqueous solution containing an alkali metal chlorite or alkaline earth metal chlorite in a concentration of at least 100 grams per liter along with sodium or potassium hydroxide at a concentration in the range of from 5 to 25 grams per liter. The solution is then maintained at a temperature of from 80.degree. to 200.degree. F. while contacting the copper foil to deposit the oxide layer. Another reference, U.S. Pat. No. 4,512,818 describes additives which can be used in solutions for the formation of oxide coatings. The additives control various properties of the oxide coating and improve the ability to bond copper to dielectric substrates. The additives used are soluable polymers dissolved in the solution in a minor amount. The additives reduce the formation of the oxide coating in thickness and produce greater homogeniety. The coatings are further described as being mechanically dense and strong in that they do not fracture when mechanically rubbed to yield loose granular black copper oxide powder as do the coatings in the prior art. The additives are used in the solution in an amount between 3 and 100 parts per million parts of solution.
Although improvements have been made since these early compositions allowing the production of more reliable oxide coatings, adherence between the copper and dielectric polymeric material continues to be a problem. Certain polymeric substrates such as polyimide have been particularily difficult to use due to low copper-polymer bond strength.
Furthermore, delamination and untimely failure of printed wiring boards still occurs. It is therefore highly desirable to provide oxide coatings for copper which are capable of delivering improved adhesion of the copper to the dielectric material. It is also desirable to extend the life of a printed wiring board by providing an oxide coating which will reduce delamination.
A composition is described which can be used to give copper an oxide coating in order to improve adhesion between the dielectric material and the copper. Improved adhesion to polyimide has been particularily noted. When the instant composition is used, it has been found that adhesion of the copper to the dielectric material is excellent and the bond is capable of extending the useful life of the multi-layer composite produced. In addition to this, the instant coating improves the stability of the cu/polymer bond and makes it less vulnerable to thermal stresses.